Vacuum tubes for providing a favorable atmosphere for generating and accelerating an electron beam are commonly used for such purposes as exciting areas on a phosphorescent screen to allow television viewing. Typically, the electron beam that is generated within a vacuum tube is confined within the tube. However, in some applications it may be desirable to emit the beam from the vacuum tube for treatment of surfaces.
U.S. Pat. No. 5,414,267 to Wakalopulos, which is assigned to the assignee of the present invention, describes an electron beam tube that projects a stripe-like beam through a window of the vacuum tube. The beam may then be used for radiation chemistry, such as surface treatment of materials or curing of adhesives. Electrons are generated at a linear filament that is a thermionic electron emitter. A beam-forming electrode having a shape of a parabolic cylinder defines the beam as it is driven from the linear filament. The stripe-like beam is directed to an anode to project the beam through the window.
An array of electron beam devices of the type described in Wakalopulos may be arranged so that the resulting array of stripe-like electron beams cooperates to treat a wide surface. The patent teaches that a practical length of a beam emitted by such a device is in the range of 1-3 inches (25.4-76.2 mm). At the high end of this range, the thermionic filament must have a length of 76.2 mm. This would require a relatively large vacuum tube body.
U.S. Pat. No. 4,764,947 to Lesensky also teaches a tube that emits a stripe-like beam. A filament having a length corresponding to the desired length of the beam is held at the same electrical potential as a first focusing cup. A second focusing cup that is between the filament and the first focusing cup has a higher negative electrical potential than the first focusing cup. With this arrangement, undesired electron emission from selected portions of the filament are suppressed. Specifically, electron emission from the side and rear portions of the filament are suppressed, thereby substantially eliminating B-distribution from the electron distribution of the focal spot produced by the electron beam. The x-ray tube of Lesensky projects a beam having a length that corresponds to the length of the filament.
In comparison, U.S. Pat. No. 3,609,401 to Harris et al. describes an electron gun that transforms a beam of electrons having a circular cross section into a sheet beam. Electrons are emitted from a filament toward a first anode having a circular central aperture. The beam that passes through the first anode is generally circular in cross section. A pair of electrodes are on opposed sides of the circular beam. The electrodes combine to define an ellipse having open ends at the long dimension. The elliptical arrangement provides a focusing action along the short dimension, but the beam diverges in the long dimension. The patent describes the resulting beam as having a flat shape resembling an ax blade. Thus, the nonsymmetrical focusing field established by the electrodes alters the shape of the electron beam from a circular cross section as it passes through the aperture in the first electrode to a substantially rectangular cross section as it passes through the second electrode. The invention may be used for electron probe microanalysis. While the electron gun of Harris et al. operates properly for many applications, a concern is that by allowing the beam to be left free to diverge in long dimension, the resulting beam does not possess the necessary characteristics for precision treatment of surfaces.
An object of the invention is to provide an electron beam device in which a generated beam has a controlled beam length that is substantially greater than a controlled beam width.